1. Field of the Invention:
This invention relates to a non-volatile storage device, and more particularly to an EPROM (Erasable Programmable Read Only Memory) which has an improved selectivity.
2. Description of the Related Art:
Non-volatile memories are devices which are important in widening the application of general purpose LSIs; for example, they are used as EPROMs.
FIGS. 4A and 4B of the accompanying drawings show a MONOS (Metal Oxide Nitride Oxide Semiconductor) memory as a typical non-volatile storage device. The MONOS memory comprises a silicon oxide film 12, a silicon nitride film 14, a silicon oxide film 16 and a polysilicon electrode 18, which are laminated in order in a gate region of a p-type silicon substrate 10. During data writing, as shown in FIG. 4A, assume that a positive bias, e.g. Vg=10 V, is applied to the gate electrode; Vd=9 V, to a drain electrode; and Vs=0 V, to a source electrode. Then electrons from the silicon substrate 10 are captured and accumulated in the silicon nitride film 14 through the silicon oxide film 12 (tunnel phenomenon). With the charges accumulated, a drain current does not flow unless a voltage sufficiently excessive to cancel the captured charges is applied. Therefore the condition where upon this drain current flows or does not flow corresponds to 1 bit of data.
During erasing, as erasing, as shown in FIG. 4B, assume that a negative bias, e.g. Vg=-6 V, is applied to the gate electrode; Vd=9 V, to the drain electrode; and Vs=0 V, to the source electrode. Then holes cancel the electrons captured and accumulated in the silicon nitride film 14 through the silicon oxide film 12, so that a drain current will flow.
Thus, in the conventional MONOS memory, data is stored and erased by injecting electrons and holes into the silicon nitride film 16. In the event that a bias is applied to the gate electrode, the electric field becomes gradually weaker toward the drain electrode obliquely from the source electrode as indicated by arrows in FIG. 3. Therefore, during storing and erasing data, electrons and holes are injected only in a predetermined area of the drain region rather than the entire area of the silicon nitride film 14 so that the remaining area of the silicon nitride film is kept in a fresh condition. Injection of electrons and holes into this predetermined area makes it possible to keep the selector transistor function of the storage device.
Namely, in the case where holes are captured in the entire area of the silicon nitride film during erasing, if the holes are excessively injected (this might be caused due to the staggering characteristics at the time of production), a drain current would normally flow even in the absence of a bias to the gate electrode. As a result, because of the fact that it is composed of a plurality of MONOS elements, an EPROM does not function as a selector transistor.
Thus to function as an EPROM, the storage device requires that electrons or holes should not be captured near the source region of the silicon nitride film. However, in the conventional storage device, since electrons and holes are injected with a large quantity of energy from near the drain, there is a slight probability that the electrons and holes would move toward the source and be captured.
If the channel of the gate is lengthened, areas devoid of captured electrons or holes would be formed, but reduction in the size of the device cannot be achieved.